1. Field of the Invention
The present invention relates to a method of manufacturing an SZ-slot type optical fiber cable in which optical fibers are stored in spiral grooves which are provided in an SZ slotted body so as to turn over alternately.
2. Description of the Related Art
An optical fiber cable in which spiral grooves are provided in a columnar member so an to store optical fibers therein is well known as protecting delicate optical fibers from various kinds of stresses and being capable of storing optical fibers at a high density.
In addition, a slotted body in which the directions of spiral grooves are made to turn over alternately is called an SZ slotted body, and adopted as one type of slot-type optical fiber cables, by which only desired optical fiber can be extracted easily only by removing an outer jacketing member in an intermediate portion without cutting the cable. An example of such a slotted body in shown in FIGS. 5 and 6.
FIG. 5 shows an example of an SZ slotted body constituted by a tensils strength member 22 formed from a steal wire, a steel strand, an FRP rod or the like, and an optical fiber storage body 23 formed from a plastic material such as polyethylene and provided with spiral grooves 23a turning over alternately in its outer circumferential surface. In accordance with necessity, there is a case where a columnar rod formed from metal such as aluminum which is completely solid to its center portion so as to omit the central tensile strength body and which is provided with spiral grooves formed in its surface so as to turn over alternately is adopted as an SZ slotted body.
In addition, optical fibers are stored in the grooves. As for the formation of the optical fibers in a groove, there are various forms as shown in FIG. 6. The reference numeral 24 represents a bundle of a plurality of coated glass fibers wound with yarn, tape or the like, 25 represents a stack of optical fiber ribons, each ribbon being formed of a plurality of coated glass fibers arranged side by side and collectively coated so as to be made into a ribbon, 26 represents a coated glass fiber disposed in a groove, and 27 represents a unit of coated glass fibers, each unit being two coated glass fibers further coated collectively. In this way, various forms of optical fiber formation can be considered. Although different forms of optical fiber formation are shown in the respective grooves in FIG. 6 by way example, optical fibers stored in the grooves of one optical fiber cable are, in most of cases, in the same formation. In addition, in FIG. 6, the reference numeral 29 represents a winding tape, and 29 represents an outside jacketing of a plastic material or the like.
To manufacture such a kind of SZ-slot type optical fiber cable, such an assembling unit as shown in FIG. 7 has been generally used. In FIG. 7, the reference numeral 31 represents an SZ slotted body; 32, spiral grooves formed in the SZ slotted body so as to turn over alternately; 33, supply bobbins; 34, optical fibers; 35 and 36, guide disks rotating around the axis while turning ever alternately; and 37, guide pipes for guiding the optical fibers. In the manufacturing method using this assembling unit, the optical fibers 34 supplied from the supply bobbins 33 are passed through the guide disks 36 and 35 rotating around the axis while turning over alternately, passed through the guide pipes 37, and introduced into the spiral grooves formed in the SZ slotted body and turned over alternately.
The guide pipes 37 are attached to the guide disk 35, so that the pipes 37 rotate around the axis together with the guide disk 35 synchronously with the rotation of the spiral grooves 32 around the axis as the SZ slotted body 31 advances in the direction of the arrow. One of the end portions of each of the guide pipes 37 is disposed above one of the spiral grooves to introduce an optical fiber or stack of optical fiber ribbons extracted therefrom into the spiral groove.
In such a conventional manufacturing method, in a section where the spiral grooves do not turn over alternately, because the supply bobbins 33 do not rotate around the axis, the guide disk 35 rotates in one direction so that the optical fibers positioned between the supply bobbins 33 and the guide disk 35 are wound on the body around the axis at an angle that corresponds to the rotation angle of the guide disk 35. Since the intermediate portions of the optical fibers are guided by the guide disk 36, there is no fear that the optical fibers touch the SZ slotted body or the like in the intermediate portions if the rotation angle is not so large before alternate turn-over, However, if the rotation angle is so large before turn-over, the optical fibers may be wound on the SZ slotted body in a position between the supply bobbins 33 and the guide disk 36 or between the guide disk 36 and the guide disk 35.
In addition, most of SZ slotted bodies have tensile strength wires in their center as shown in FIG. 5. An such a tensile strength wire, representatively, a single steel wire or steel strands in which a plurality of steel wires twisted are used. However, local axial torsion dwells in these steel wires or steel strands as scattering when they are manufactured. This torsion is released when the SZ slotted body is extended, and even if the SZ slotted body is extracted and advanced straight, it occurs that the SZ slotted body rotates around the axis suddenly when it comes to the above-mentioned place where the torsion is released. And then, it is difficult to forecast when this rotation caused by the release of the torsion will occur.
When there occurs irregular rotation around the axis of the SZ slotted body caused by the release of local torsion of the tensile strength wire, there is a case where the optical fibers are wound around the SZ slotted body irregularly, so that the optical fibers are injured, or given large stress so as to provide an adverse influence on their transmission lose. Although working at a low speed can be also considered in order to prevent such a case, it is not preferable economically.